1. Field of the Invention
The present invention relates to a scanner, and more particularly, to an optical scanner that doesn""t need an automatic gain control unit.
2. Description of the Prior Art
When scanning a document with a typical scanner, the scanning module produces an induced voltage that is related to the amount of light received from the document. This induced voltage is passed on to an automatic gain control unit for scaling, and is then sent on to an A/D (analog to digital) converter where it is converted into a digital image signal.
Please refer to FIG. 1. FIG. 1 is a function block diagram of a prior art scanner 10. The scanner 10 comprises a scanning module 12 for scanning a document and generating corresponding image signals, a cathode-ray tube 16 for providing light to the document, a power supply 18 for supplying power to the cathode-ray tube 16, and a control circuit 20 for controlling the operations of the scanner 10.
The scanning module 12 comprises a plurality of color detectors 14, a shift register 22, an output unit 28, an automatic gain control unit 30 and an A/D (analog to digital) converter 32. The color detectors 14 are used to detect light of different color and generate an electric charge that corresponds to the amount of light received. The shift register 22 has a plurality of serially connected storage units and an output port 26 into which the contents of the storage units can be sequentially placed. The output unit 28 is electrically connected to the output port 26 of the shift register, receiving the electric charge held in the storage units 24 and outputting a corresponding voltage. The automatic gain control unit 30 receives and adjusts the voltage from the output unit 28, passing the adjusted voltage on to the A/D converter 32. The A/D converter converts the analog voltage from the automatic gain control unit 30 into a digital image signal.
The color detectors 14, shift register 22 and output unit 28 of the scanning module 12 are installed in a single light converting module 34. The number of storage units 24 and the number of color detectors 14 are in proportion. Each color detector 14 is electrically connected to one or more storage units 24. The storage units 24 are used to store the electric charge generated by the connected color detectors 14. This stored charge can be sequentially moved to the output unit 28.
The power supply 18 of the scanner 10 continuously supplies power to the cathode-ray tube 16, and the control circuit 20 controls the exposure time, which is the same for all of the color detectors 14, to control the amount of induced charge generated by the color detectors. The shift register 22 sequentially sends the induced charge in the storage units 24 to the output unit 28 to create an induced voltage. However, the induced voltage cannot be directly sent to the A/D converter 32 to be converted into a digital image signal. The induced voltage must first be adjusted to lie within the working range of the A/D converter 32. Consequently, the induced voltage must be adjusted by the automatic gain control unit. This automatic gain control unit is an extra expense in the scanner circuitry. Furthermore, under identical exposure conditions, the induced charge generated by the color detectors 14 under red, green and blue light is not the same, and hence the induced voltages are also not the same. Generally, red light corresponds to about 6 to 7V, green light corresponds to about 8V, and blue light corresponds to about 2 to 3V. In order to account for this factor the design of the automatic gain control unit 30 becomes even more complicated, which also increases costs.
The control circuit 20 controls whether or not the color detectors 14 will receive light from the cathode-ray tube 16 and convert it into induced charge. In the prior art, the power supply supplies power to the cathode-ray tube 16 even if the color detectors 14 are in a state where they are incapable of receiving light. Thus, a lot of power is wasted providing illumination when it is not needed. Consider, for example, the prior art scanners which use CCDs as the color detectors 14. Due to the electrical characteristics of CCD circuits, the next exposure has to wait until the charge received from the previous exposure has been sequentially converted into the corresponding induced voltage, passed on to the automatic gain control unit 30 and the A/D converter 32, and then transmitted to memory 36. During this time, the cathode-ray tube continues to provide light which cannot be used by the CCD.
When scanners are developed for high-resolution applications, the number of cells in the CCD increases linearly. The time required, then, for the CCD to transmit charge also increases linearly, and so the power wasted by the cathode-ray tube 16 becomes even more serious. Nearly every country in the world has begun implementing energy-management standards for electronic products, and these standards are becoming more and more strict. Reducing the power consumption of the cathode-ray tube 16 becomes an important theme.
It is therefore a primary objective of the present invention to provide a scanner to solve the above mentioned problems.
In a preferred embodiment, the present invention provides an optical scanner comprising:
a scanning module comprising a plurality of color detectors for detecting light of different colors when scanning a document to generate corresponding electric charge;
a cathode-ray tube for providing light to the document;
a power supply for supplying power to the cathode-ray tube; and
a control circuit for controlling operations of the optical scanner and the power supply;
wherein the control circuit controls an exposure time of each of the color detectors according to the sensitivity of the color detector to its corresponding color of light so that the amount of electric charge generated by each of the color detectors during each exposure can be controlled.
It is an advantage of the present invention that the scanner does not need to use an automatic gain control unit to scale the induced voltages. Hence, the cost of the components and assembly of the scanner can be largely reduced. A lot of time can also be saved in high-resolution applications because adjusting the induced voltages for wide rows of pixels is no longer required. In addition, the power supply supplies power to the cathode-ray tube 46 in a discontinuous manner, further reducing the power consumption of the scanner.
This and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the various figures and drawings.